In the ongoing policy debate about the future of European Union defense, the “Weimar Three” (France, Germany, and Poland) have found themselves in direct conflict with Britain. London, which staunchly opposes any policy position that could undermine the NATO alliance, has declared that one of the biggest goals of EU defense advocates — creating a permanent EU military headquarters — is a “red line.”

However, the three pro-HQ nations have forwarded a renewed proposal and are attempting to push the controversial issue forward against Britain’s will using a little-used EU legal clause. The EU HQ debate, which by incremental steps represents the future of EU and European defense cooperation — as opposed to NATO — has thus started to expose deep divisions in policy direction.

In July, I reported that due to the UK’s power of veto in the European Council, proposed plans for an EU military headquarters were doomed to fail. Facing this inevitable veto, the Weimar Three have now brought a new weapon to their fight for a HQ — Permanent Structured Cooperation (PSC).

In PSC, pioneering member states who meet certain military criteria can forge an “EU-level” defense structure without unanimity, as long as a large enough group (eight states) are willing to move ahead for the initial stages.

The rule was created to free member states from the slow-moving negotiations of the EU when there is an urgent policy need. Yet, in invoking its potential use for something as existential and contentious as a military HQ, the Weimar Three are raising the stakes considerably by bypassing a major member state on a core policy issue.

Will the invocation of PSC work? Legally, the answer is probably yes, though some internal wrangling could still be needed in the other two branches of the EU — the European Parliament and Commission. Yet in diplomatic terms, the cost-benefit equation is rather different.

PSC — and its civic policy equivalent of “enhanced cooperation” — is quite controversial. Many representatives have already expressed a concern that a de facto opt-out of the EU’s consensus-based negotiation process “is not necessarily the image we want to give of the EU."

Given this, the Weimar Three have to decide if they are willing to bring the PSC clause under wider scrutiny to achieve their current aims. Given that several of these same states are pushing for closer economic integration through the enhanced cooperation clause, an immensely controversial debate, they may not want any negative publicity for this law just now.

Yet perhaps more pragmatically, they need to decide if it is worth causing a rift with London over this issue. The UK’s combination of experienced military leaders and quickly deployable armed forces certainly gives them an influential voice on defense issues, and Britain also accounts for 25% of all European defense spending. The UK even hosts the command infrastructure of the EU’s most ambitious military mission – EUNAVFOR.

The UK has a “controlling share” of several key EU defense initiatives. As the battle lines over the EU HQ are drawn, the Weimar Three will have to consider if they are willing to risk a long-term split between Britain and the EU over defense cooperation and what it will cost if they do.